1. Field of the Invention
This invention relates in general to methods and systems for accurately measuring the internal dimensions of a tube within a borehole and in particular to a method and system for measuring and storing maximum and minimum variations in internal dimensions of a tube within a borehole over a predetermined time interval. Still more particularly, the present invention relates to a system for providing highly accurate measurement of borehole tube internal dimensions utilizing a large number of closely spaced detectors.
2. Description of the Prior Art
The modern petrochemical industry utilizes a large number of tubes which are designated primarily for the circulation of fluids within a wellbore. Notable examples of such tubes are those utilized for well drilling and fluid circulation in an oil well. In actual practice, it is often necessary to measure the ovalization or corrosion of either the internal or external surfaces of these tubes to ensure that tubing which is about to fail is replaced. Such measurements are often carried out in atmospheres which may deleterious or difficult to reach, for example at the bottom of an oil well in the case of tubing strings. In order to carry out these measurements, provisions must be made for a reliable and accurate measurement which operates in a simple and straightforward manner.
Early systems for measuring the shape of a cylindrical tube within a wellbore were primarily impression-taking devices of a mechanical type which may produce electrical signals as a result of mechanical contact between the measuring device and a dimension of the tubing. Typically such devices utilizing an engraving stylus on a substrate for the purpose of marking a curve which corresponds to ovalization of a cylinder or to surface defects within the cylinder. These devices are complicated by reason of the fact that they require high precision mechanical transmission systems and the fact that a mechanical marking operation impairs the sensitivity of the measurement over time. Moreover, such devices do not immediately deliver usable information since the device must be removed to the surface of a wellbore for analyzation after the measurements have been taken.
Electromechanical devices utilizing multiple feeler arms have been proposed which overcome this mechanical device problem. One example is set forth in U.S. Pat. No. 4,186,494, Peruchon Edouard et al., issued Feb. 5, 1980. The transducers set forth within the above-referenced United States patent are quite bulky and only a limited number of fingers may be placed around a sonde owing to the space necessary for corresponding transducers within the sonde.
More recently, U.S. Pat. No. 4,559,709, issued to Beseme et al., Dec. 24, 1985, discloses a technique whereby multiple sensing fingers are provided, each movable with respect to a sonde. A mobile member attached to each sensing finger and movable in response to movement of the finger is placed within an electric coil and is utilized to vary the inductance of that coil in response to variations in the internal dimensions of a tube. Thereafter, the device may be periodically interrogated from the surface, utilizing a signal transmitted over a conductive cable, and the output of each transducer is then transmitted to the surface, providing an indication of defects or irregularities in the internal dimensions of a tube within a borehole.
While each of these systems represents an adequate method for measuring the internal dimensions of a tubular member within a borehole, several shortfalls have been experienced with these devices. For example, the Beseme et al. device transmits the output of each transducer to the surface in response to an interrogation from the surface and thus defects which are located physically between two points within the tube at which an interrogation takes place will not be noted utilizing a tool of this type. Additionally, the transducers described therein, while an improvement over Peruchon Edouard et al., still require a substantial amount of space and therefore preclude the provision of a large number of sensing figures which might be utilized to provide a highly accurate indication of irregularities within the internal dimensions of a tubing member within a borehole.
It should therefore be apparent that a need exists for a method and system which permits highly accurate measurements of the internal dimensions of a tube within a borehole utilizing a large number of sensing transducers disposed circumferentially about a measurement tool.